The present invention relates to a plastic-coated optical transmission fiber in which a glass fiber is coated with an organic matter. More specifically, this invention relates to a more effective means of transmission due to said coating.
Recently, demand for long-distance optical communication has increased and further improvement in transmission characteristics of optical fiber transmission line is desirable. Conventional optical transmission fibers and the conventional method for estimating the tightness between the glass fiber and its coating have become insufficient to meet current demand.
The particular type of coating material used in an optical fiber transmission line influences its transmission characteristics. It has been reported that when temperature changes over a wide range, the shrinking and expanding of the coating material cause a microbending of the glass fiber and thereby causes a deterioration of transmission characteristics.
It has been difficult to obtain using standard production methods optical transmission fibers superior in transmission characteristics with good reproducibility over a wide temperature range on the basis of only the theoretically calculated forces resulting from shrinking or expanding of the coating material. Further, it has been difficult to estimate accurately the tightness between the glass fibers and the coatings.
Accordingly, there has been investigation of the influence on transmission characteristics of the tightness between the coating material and glass fiber and how to estimate that tightness.
In a conventional method of estimating tightness, the "drawing" force with which a glass fiber is pulled out of an optical transmission fiber is measured so as to determine "fastening" force with which the coating material fastens to the glass fiber. Tightness can also be estimated from the quantity of shrinkage of the coating material on the basis of a heat-cycle test in a temperature range from a low temperature to a high temperature (for example, from -40.degree. C. to +60.degree. C.).
However, in practice, even in cases where it was estimated that the degree of tightness should be suitable in accordance with conventional estimating methods, abnormality often occurred in transmission loss when an optical transmission fiber was used at low or high temperatures.
Accordingly, it has been desired to develop an optical transmission fiber and an estimation method, in which a glass fiber and a coating material are fabricated so as to be in close contact with each other in a manner providing a good transmission characteristic over a wide temperature range.
Optical fibers for communication are formed in a manner so that a glass base material (preform) is spun and then coated with a macro molecular material. A generally-used optical transmission fiber, made from a glass fiber of silica glass, fluoride glass, or the like, has both a central core and an outside clad. The glass fiber, both the core and the clad, is coated with a soft layer. The outside of the glass fiber coated with the soft layer is further coated with a hard layer so as to form an optical transmission fiber having a single core.
The soft layer acts as a cushion against the glass fiber and is made of a soft resin. Specifically, the soft resin may be thermosetting silicone, ultraviolet (hereinafter abbreviated to "UV") curable silicone, UV curable urethane acrylate, UV setting epoxy acrylate, UV setting ester acrylate, or the like. The hard outside layer protects the glass fiber from the outside of the soft layer and is made of a stiff resin. The stiff resin may be extrusion resin such as polyamide, polyester, ABS resin, polyacetal resin, or the like, or any kind of UV curable resin. Those coating materials are often used in the colored state. When used in such a manner, either or both of the materials for the soft layer and the hard layer is colored. Sometimes, a colored layer is provided outside the hard layer or is interposed between the soft layer and the hard layer.
Investigation has shown that the conventional coating materials have been used in various combinations. That is, for the soft layer, generally a material is used that has a glass transition temperature lower than -50.degree. C. and a Young's modulus lower than 0.5 Kg/mm.sup.2 at ordinary temperatures. For the hard layer a material is generally used that has a glass transition temperature higher than an ordinary temperature (0.degree..about.20.degree. C.) and a Young's modulus higher than 30 Kg/mm.sup.2 at ordinary temperatures. By variously combining those materials, it is possible to improve the transmission characteristics.